1. Field of the Invention
The present invention relates to zoom lens systems, interchangeable lens apparatuses, and camera systems. In particular, the present invention relates to: compact and lightweight zoom lens systems having, as well as excellent optical performance, short overall length, and being able to be held with small lens barrel; and interchangeable lens apparatuses, and camera systems, each employing the zoom lens system.
2. Description of the Background Art
In recent years, interchangeable-lens type digital camera systems (also referred to simply as “camera systems”, hereinafter) have been spreading rapidly. Such interchangeable-lens type digital camera systems realize: taking of high-sensitive and high-quality images; high-speed focusing and high-speed image processing after image taking; and easy replacement of an interchangeable lens apparatus in accordance with a desired scene. Meanwhile, an interchangeable lens apparatus having a zoom lens system that forms an optical image with variable magnification is popular because it allows free change of focal length without the necessity of lens replacement.
Zoom lens systems having excellent optical performance from a wide-angle limit to a telephoto limit have been desired as zoom lens systems to be used in interchangeable lens apparatuses. Various kinds of zoom lens systems each having a negative lens unit located closest to an object side, and a multiple-unit construction have been proposed.
For example, International Patent Publication No. 2008-072466 discloses a variable magnification optical system having a three-or-more unit construction of negative, positive and negative lens units. In the optical system, the interval between the first lens unit and the second lens unit decreases when the magnification varies from a wide-angle limit to a telephoto limit. The first lens unit includes at least one negative lens and at least one positive lens. The second lens unit is composed of one positive lens and one negative lens, and has at least one aspheric surface. There is defined the condition setting forth a distance from the most object side lens surface to an image surface, a difference in refractive index of each lens and a difference in Abbe number of each lens in the second lens unit, and a distance from the most image side lens surface to the image surface.
Japanese Laid-Open Patent Publication No. 2001-116992 discloses a zoom lens having a four unit construction of negative, positive, negative and positive lens units. In the zoom lens, when the magnification varies from a wide-angle limit to a telephoto limit, the first lens unit moves with locus of a convex to the image side, the second lens unit moves to the object side so that the interval between the second lens unit and the first lens unit decreases, the third lens unit moves to the object side so that the interval between the third lens unit and the second lens unit increases, and the fourth lens unit moves to the object side so that the interval between the fourth lens unit and the third lens unit decreases. The third lens unit is composed of one negative lens, and the negative lens has an aspheric surface.
In each of the variable magnification optical system and the zoom lens disclosed in the above-mentioned patent literatures, the construction of the lens unit contributing focusing makes it difficult to reduce the overall length of lens system although optical performance is maintained at a certain level. In particular, increase in size of a lens barrel holding such variable magnification optical system or zoom lens cannot be prevented.